1. Field of the Invention
This invention relates to a method of removing sulfur and nitrogen oxides in a waste gas and more particularly, it is concerned with an improved method of removing sulfur and nitrogen oxides contained in a combustion waste gas from a boiler, etc. by adding ammonia to the waste gas and passing the thus-mixed gases through a moving bed formed of a carbon-containing adsorbent in a transverse direction at a temperature of from room temperature to 180.degree. C.
2. Description of the Prior Art
A method has been known comprising adding ammonia to a waste gas containing sulfur oxides, nitrogen oxides, steam and oxygen, passing the waste gas through a single moving bed of a carbon-containing adsorbent and thus removing simultaneously the sulfur and nitrogen oxides at a relatively low temperature (e.g. room temperature to 180.degree. C.) by the adsorption and catalytic action of the carbon-containing adsorbent. In this method, sulfur compounds are separated as sulfuric acid, ammonium sulfate or ammonium hydrogensulfate and nitrogen compounds are separated as nitrogen, ammonium nitrate or ammonium nitrate by the catalytic action of the carbon-containing adsorbent. In the temperature range of from room temperature to 180.degree. C., however, the reaction of ammonia and sulfur oxides precedes that of ammonia and nitrogen oxides, so the prior art method wherein these reactions are simultaneously carried out in a single moving bed has the disadvantage that the removal efficiency of nitrogen oxides is not increased and the consumption of ammonia is markedly increased. Thus, it is considered to raise the temperature of a waste gas at the sacrifice of boiler efficiency or to raise the temperature before feeding to the reaction bed by the use of a new heat source, but this is not favourable economically because of the need to raise the temperature by about 50.degree. C.
In another known system as shown in FIG. 1, two moving beds 4 and 5 are arranged in parallel and carbon-containing adsorbent 2 is separately fed to first moving bed 4 and second moving bed 5, while waste gas 1 is passed through first moving bed 4 and then through second moving bed 5 in a transverse direction thereto. In the first moving bed, the most part of sulfur oxides is removed by the adsorption action of the carbon-containing adsorbent and in the second moving bed, nitrogen oxides are separated as nitrogen, ammonium nitrate or ammonium nitrite by reaction with ammonia 3 added before the second moving bed and by the catalytic action of the carbon-containing adsorbent. During the same time, the adsorbent whose activity is lowered is regenerated in regenerator 6 and if necessary, ammonia 3' can be added to the waste gas before first moving bed 4. This is apparently a reasonable process, but the quantity of the carbon-containing adsorbent to be moved for regeneration is markedly large, since the carbon-containing adsorbent is separately introduced into two moving beds arranged in parallel and regeneration of the carbon-containing adsorbent whose activity is lowered due to the deposition of ammonium nitrate or ammonium nitrite in the second moving bed is required in addition to that of the first moving bed.
In an apparatus for removing sulfur and nitrogen oxides by a dry process using carbon-containing adsorbents, the proportion of the cost of carbon-containing adsorbent is large compared to the operation cost of the apparatus and accordingly, it may safely be said that the former is the key factor in this dry process system. Furthermore, the regeneration of carbonaceous adsorbents is generally carried by heating using fuels such as COG, fuel oils and the like and the fuel cost is not negligible. Therefore, the above described method cannot avoid the drawback that a larger quantity of carbon-containing adsorbent is moved and regenerated, thus resulting in increase of the quantity of carbon-containing adsorbent consumed or lost and the quantity of a fuel consumed and thus raising remarkably the operation cost.